Liquid crystal materials are useful for electronic displays because light traveling through a layer of liquid crystal (LC) material is affected by the anisotropic or birefringent value (.DELTA.n) of the LC material which in turn can be controlled by the application of a voltage across the LC. Liquid crystal displays (LCDs) are commonly used in applications such as avionic cockpit displays, portable computers, calculators, etc.
Informational data in typical liquid crystal displays is presented in the form of a matrix array of rows and columns of numerals or characters which are generated by a number of segmented electrodes arranged in a matrix pattern. The segments are connected by individual leads to driving electronics which apply a voltage to the appropriate combination of segments and adjacent LC material in order to display the desired data and/or information by controlling the light transmitted through the liquid crystal material.
Contrast ratio is one of the most important attributes considered in determining the quality of both normally white (NW) and normally black (NB) liquid crystal displays. The contrast ratio in a normally white display is determined in low ambient conditions by dividing the "off-state" light transmission (high intensity white light) by the "on-state" or darkened transmitted intensity. For example, if the "off-state" transmission is 200 fL at a particular viewing angle and the "on-state" transmission is 5 fL at the same viewing angle, then the display's contrast ratio at that particular viewing angle is 40 or 40:1 for the particular "on-state" driving voltage utilized.
Accordingly, in normally white LCDs the primary factor adversely limiting the contrast ratio is the amount of light which leaks through the display in the darkened or "on-state". In a similar manner, in normally black displays, the primary factor limiting the contrast ratio achievable is the amount of light which leaks through the display in the darkened or "off-state". The higher and more uniform the contrast ratio of a particular display over a wide range of viewing angles, the better the LCD.
Normally black (NB) twisted nematic displays typically have better contrast ratio contour curves or characteristics then do their counterpart NW displays in that the NB image can be seen better at large viewing angles. However, NB displays are much harder to manufacture than NW displays due to their high dependence on the cell gap or thickness "d" of the liquid crystal layer as well as on the temperature of the liquid crystal material itself. Accordingly, a long-felt need in the art has been the ability to construct a normally white display with high contrast ratios over a large range of viewing angles, rather than having to resort to the more difficult to manufacture NB display to achieve these characteristics.
What is generally needed in NW displays is an optical compensating or retarding element(s), i.e. retardation film, which introduces a phase delay that restores the original polarization state of the light, thus allowing the light to be substantially blocked by the output polarizer in the "on-state". Optical compensating elements or retarders are known in the art and are disclosed, for example, in U.S. Pat. Nos. 5,184,236, 5,196,953, 5,138,474, and 5,071,997, the disclosures of which are hereby incorporated herein by reference.
FIG. 1 is a contrast ratio curve graph for a prior art normally white twisted nematic light valve including a rear linear polarizer having a transmission axis oriented in a first direction, a front or light exit linear polarizer having a transmission axis defining a second direction wherein the first and second directions are substantially perpendicular to one another, a liquid crystal material having a cell gap "d" of about 5.86 .mu.m and a birefringence (.DELTA.n) of about 0.084 at room temperature, a rear buffing or orientation film buffed in the second direction, and a front orientation film buffed in the first direction. The temperature at which FIG. 1 was developed was about 34.4.degree. C. This light valve did not include a retarder.
The contrast ratio curves of FIG. 1 were plotted utilizing a 6.8 volt "on-state" driving voltage, a 0.2 volt "off-state" or V.sub.OFF voltage, and by conventionally backlighting the display with white light. As can be seen in FIG. 1, the viewing zone or envelope of the light valve while being fairly broad horizontally in the lower vertical region becomes narrowed or constricted in the positive vertical viewing region. For example, at positive 20.degree. vertical, the 10:1 and greater contrast ratio region extends horizontally over only a total of about 70.degree. while at -20.degree. vertical, this same 10:1 contrast ratio zone extends over a horizontal total of about 100.degree.. Therefore, because of the non-uniform or skewed shape of the viewing zone or envelope shown in FIG. 1, it is evident that viewers in the positive vertical viewing region will have difficulty viewing displayed images at medium and large horizontal viewing angles such as about .+-.40.degree.. This graph is illustrative of the common problems associated with typical normally white liquid crystal displays in that their contrast ratios are limited at increased horizontal and vertical viewing angles.
FIG. 2 is a driving voltage versus intensity (fL) plot of the prior art light valve described above with respect to FIG. 1, this plot illustrating the gray level behavior of this light valve. The various curves represent horizontal viewing angles from about -60.degree. to +60.degree. along the 0.degree. vertical viewing axis.
Gray level performance and the corresponding amount of inversion are important in determining the quality of an LCD. Conventional liquid crystal displays typically utilize anywhere from about 8 to 64 different driving voltages. These different driving voltages are generally referred to as "gray level" voltages. The intensity of light transmitted through the pixel(s) or display depends upon the driving voltage utilized. Accordingly, conventional gray level voltages are used to generate dissimilar shades of color so as to create different colors when, for example, the shades are mixed with one another.
Preferably, the higher the driving voltage in a normally white display, the lower the intensity (fL) of light transmitted therethrough. Likewise then, the lower the driving voltage, the higher the intensity of light reaching the viewer. The opposite is true in normally black displays. Thus, by utilizing multiple gray level driving voltages, one can manipulate either a NW or NB liquid crystal display to emit desired intensities and shades of light/color. A gray level V.sub.ON is generally known as any driving voltage greater than V.sub.th (threshold voltage) up to about 5-6.5 volts.
Gray level intensity in LCDs is dependent upon the display's driving voltage. It is desireable in NW displays to have an intensity versus driving voltage curve wherein the intensity of light emitted from the display or pixel continually and monotonically decreases as the driving voltage increases. In other words, it is desireable to have gray level performance in an NW pixel such that the intensity (fL) at 6.0 volts is less than that at 5.0 volts, which is in turn less than that at 4.0 volts, which is less than that at 3.0 volts, which is in turn less than that at 2.0 volts, etc. Such desired gray level curves across wide ranges of view allow the intensity of light reaching viewers at different viewing angles via the pixel(s) or display to be easily and consistently controlled.
Turning again to FIG. 2, the intensity versus driving voltage curves illustrated therein of the FIG. 1 light valve having no retardation film are undesireable because of the inversion humps present in the areas of the curves having driving voltages greater than about 3 or 3.2 volts. The intensity aspect of the curves monotonically decreases as the driving voltage increases in the range of from about 1.6-3.0 volts, but at a driving voltage of about 3.2 volts, the intensities at a plurality of viewing angles begin to rise as the voltage increases from about 3.2 volts up to about 6.8 volts. Such rises in intensity as the driving voltage increases are known as "inversion humps". Inversion humps lead to the display or light valve emitting different colors via the same pixel at different viewing angles for the same driving voltage. Clearly, this is undesirable. Whilte the inversion humps of FIG. 2 include only rise portions, inversion humps often include both rise and fall portions as will be appreciated by those of ordinary skill in the art, thus enabling the "inversion humps" to actually look like humps.
A theoretically perfect driving voltage versus intensity (fL) curve for an NW display would have a decreased intensity (fL) for each increase in gray level driving voltage at all viewing angles. In contrast to this, the inversion humps of FIG. 2 represent large increases in intensity of radiation emitted from the light valve for each corresponding increase in gray level driving voltage above about 3.2 volts. Accordingly, it would satisfy a long-felt need in the art if a normally white liquid crystal display could be provided with no or little inversion.
U.S. Pat. No. 5,184,236 discloses an NW display including a pair of retardation films provided on one side of the LC layer, these retardation films having retardation values of about 300 nm or greater. The viewing characteristics of the LCDs of this patent could be improved upon with respect to contrast ratio, inversion, and uniformity as well as the position of the viewing zone by utilizing retarders of different values and orientations. Furthermore, it is felt that such improvements may be achieved with a reduced number of retardation films thus reducing the cost and complexity of the display.
The parents of this application, i.e. Ser. Nos. 08/167,652 and 08/235,691 incorporated herein by reference, provide for NW displays with a pair of retardation films having retardation values of about 80-200 nm. While the different embodiments of Ser. Nos. 08/167,652 and 08/235,691 provide excellent results with respect to viewing characteristics, the disclosure of this application allows improved viewing characteristics in the vertical viewing regions while sacrificing certain viewing characteristics at other viewing angles.
FIG. 3 illustrates the angular relationships between the horizontal and vertical viewing axes and angles described herein relative to a liquid crystal display and conventional LCD angles .o slashed. and .THETA.. The +X, +Y, and +Z axes shown in FIG. 3 are also defined in other figures herein. Furthermore, the "horizontal viewing angles" (or X.sub.ANG) and "vertical viewing angles" (or Y.sub.ANG) illustrated and described herein may be transformed to conventional LCD angles: azimuthal angle .o slashed.; and polar angle .THETA., by the following equations: EQU Tan (X.sub.ANG)=Cosine (.o slashed.).multidot.Tan (.THETA.) EQU Sine (Y.sub.ANG)=Sine (.THETA.).multidot.Sine (.o slashed.)
or EQU Cosine (.THETA.)=Cosine (Y.sub.ANG).multidot.Cosine (X.sub.ANG) EQU Tan (.o slashed.)=Tan (Y.sub.ANG).div.Sine (X.sub.ANG)
The term "rear" when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing zones, and orientation films means that the described element is on the backlight side of the liquid crystal material, or in other words, on the side of the LC material opposite the viewer.
The term "front" when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing zones and orientation films means that the described element is located on the viewer side of the liquid crystal material.
The LCDs and light valves herein include a liquid crystal material with a birefringence (.DELTA.n) of 0.084 at room temperature, Model No. ZLI-4718 obtained from Merck.
The term "retardation value" as used herein means "d..DELTA.n" of the retardation film or plate, wherein "d" is the film thickness and ".DELTA.n" is the film birefringence.
The term "interior" when used herein to describe a surface or side of elements (or an element itself), means the side or surface closest to the liquid crystal material.
The term "light valve" as used herein means a liquid crystal display including a rear linear polarizer, a rear transparent substrate, a rear continuous pixel electrode, a rear orientation film, an LC layer, a front orientation film, a front continuous pixel electrode, a front substrate, and a front polarizer (without the presence of color filters and driving active matrix circuitry such as TFTs). Such a light valve may also include a retardation film(s) disposed on either side of the LC layer as described with respect to each example and embodiment herein. In other words, a "light valve" may be referred to as one giant pixel without segmented electrodes.
It is apparent from the above that there exists a need in the art for a normally white liquid crystal display wherein the viewing zone of the display has both high contrast ratios and little or no inversion over a wide range of viewing angles, the viewing zones position being shiftable to different vertical regions so as to allow viewers at such predetermined viewing angles (e.g. positive vertical viewing angles) to be able to satisfactorily view the displayed image.
This invention will now be described with respect to certain embodiments thereof, accompanied by certain illustrations wherein: